This section provides background information related to the present disclosure which is not necessarily prior art.
Switching converters are commonly used in the field of power conversion for many reasons, including their high efficiency and compact size. The fundamental mechanism of switching power conversion is based on the principle of binary regulation of the power flow (i.e., on and off), commonly supplemented with passive filtering using inductors and capacitors. The energy storage of capacitors and inductors allows power flow to be maintained to the output when power flow from the input is switched off. By varying the relative ratio of on and off periods, the rate of the flow of energy and associated parameters like voltage and current can be regulated. When switching frequency is sufficiently higher than the regulation bandwidth for a particular application, accurate and consistent flow of the power can be achieved (e.g., free of noise and ripple caused by discontinuous operation of the power switches). These methods, often called Pulse Width Regulation and Switching Mode Regulation, are well known in the art.
Higher switching frequencies in switching converters typically provide several advantages. For example, the required size of the output filter is reduced, the amplitude of undesired variable components of the output voltage (e.g., voltage ripple) is reduced and regulation bandwidth can be increased. However, the process of switching the flow of power in a switching converter is a source of additional power losses, commonly referred to as switching losses. Accordingly, increasing the frequency of switching increases the number of switching transitions that occur and increases the total switching losses over a given time period. As a result, switching converters are typically designed to balance the advantages provided by increased switching frequency with the burden of reduced efficiency due to extra switching losses.
There are numerous applications that benefit from (or require) high regulation bandwidth in comparison with practically achievable switching frequency for a particular type of power transistor. An example of such application is a high power, high frequency inverter used in electric motor drives. In such an application, producing an appropriately filtered sinusoidal output voltage from an on/off input sequence may require a relatively high switching frequency that generally results in poor efficiency. This situation is particularly common in applications with high voltage levels, as appropriately rated components typically have slow switching characteristics and produce significant amount of energy loss with each switching transition.
Multi-level converters are sometimes used to overcome such limitations. Multi-level converters reduce the requirement for high switching frequency by producing more than two levels of discrete voltages in the process of regulation. Multi-level converters permit fast and accurate regulation with fewer transitions (i.e., with a lower switching frequency). There are numerous examples of multi-level converters. Among the common types of multi-level converters are the diode clamped, the capacitor clamped and the cascaded multi-level converters shown in FIGS. 1, 2 and 3, respectively.
These multi-level converters perform their role of reducing the need for high frequency switching transistors, but also allow processing of higher voltages than the voltage rating of the individual components of the multi-level converter. This is possible because of the multi-cell arrangement, in which overall voltage stress is divided between multiple cells forming a stack between the two input rails. Because of this ability, it is possible to construct multi-level converters capable of processing higher voltages than could otherwise be processed with components having a particular rating or to construct multi-level converters using lower rated components than would otherwise be needed for potential savings in cost, size and efficiency.
The output current of common multi-level converters passes through a relatively large number of semiconductors in the process of power conversion. This results in larger conduction losses than if the output current passed through fewer semiconductors (as occurs in some other types of power converters). The relative importance of these losses increases in lower voltage, higher current applications. Larger conduction losses can, in some instances, offset or cancel the advantages from lower switching frequency, thus reducing the usefulness of multi-level converters.